This invention relates to the preparation of dental prostheses and more particularly to the provision of a metal-substrate dental prosthesis.
A dental restoration such as a cap or crown generally comprises a metal frame or substate, the external surface of which is coated with porcelain appropriately matching the capped or crowned tooth as well as the adjoining teeth. In the production of such a dental restoration, the decayed and/or damaged portion of the tooth is suitably removed by the dentist for reception of the desired cap or crown. Thereafter an impression is taken of such prepared tooth so that the required metal base or substrate can be formed.
The procedure generally utilized in preparing such a restoration involves the use of the so-called "lost wax technique". Upper and lower impressions of the patient's oral cavity are customarily made, for which purpose any of the usual moldable impression materials may be employed. A model or die of each impression is then made by filling the respective impression with an aqueous slurry of a gypsum or similar calcium sulfate material, and permitting the gypsum material to dry and harden. The resulting models are then removed from the respective impressions and are mounted on an articulator in a manner such that the teeth on the two models are in the same relative position as the teeth in the patient's mouth. It will be appreciated, of course, that the impressions are negatives of the patient's upper and lower tooth structures.
In this manner the conditions existing in the patient's mouth are reproduced, whether an upper or a lower restoration is required. In either case a wax pattern of the crown to be fabricated is now made by applying wax to at least the model area or areas including the missing tooth portion or portions. The net effect is that the inside surface of the resulting wax pattern matches the prepared surface of the tooth to be restored, while the outer surface of the wax pattern is shaped to fit the adjacent teeth.
The wax pattern is then removed from the model or models and invested or placed in a refractory ceramic material that has a flowable consistency such that it fills and surrounds the wax pattern. A wax sprue is formed on the pattern to provide a passage through which the subsequently vaporized wax can escape and through which the molten metal can be introduced to form the desired metal substrate. The refractory ceramic material is one that has been specially formulated to withstand the thermal shock that occurs upon sudden exposure of the same to molten metal at a high temperature.
The refractory ceramic material filling and surrounding the wax pattern is now permitted to dry and harden, whereupon heat is applied to the combination to raise its temperature to a point sufficiently high that the wax is melted and vaporized, the vaporized wax escaping through the opening formed by the sprue. There thus results a cavity mold comprising a negative replica of the crown or other restoration desired, the inside of such cavity mold corresponding to the prepared surface of the tooth to be capped.
Generally, the resulting refractory ceramic material mold is mounted in a centrifugal casting machine. The desired metal composition is then heated to melt the same; and the molten metal is poured into the cavity mold to the desired extent. Cooling and solidification of the molten metal then take place; and finally the ceramic material mold is broken away from the resulting cast crown.
Cleaning and polishing of the rough cast crown follow, together with the fitting of such crown to the prepared tooth and correction of the crown as necessary to provide a proper fit. Usually a porcelain coating is desired; and an appropriate porcelain composition is applied to the exterior of the metal crown and heat-treated in the customary manner. Finally, the finished crown is cemented in place on the prepared tooth.
Although a satisfactory metal base or substrate can be obtained by this "lost wax technique", it has been recognized that this procedure is not only laborious and technique-sensitive but is time-consuming and thus costly. Usually, at least six hours are required to produce the metal base from the impression as received from the dentist. In addition, application of the porcelain coating to the metal base involves upwards of another four hours.
Various modifications of such "lost wax technique" have been proposed from time to time. For one reason or another, however, none of these proposals has been particularly effective especially in decreasing the time required to prepare the metal coping or substrate or in materially reducing the technique-sensitivity of the restoration procedure. Thus, despite its drawbacks, this "lost wax technique" has remained as the conventional procedure for preparing a dental restoration.